Method and apparatus for transmitting and receiving reference signal

ABSTRACT

An apparatus and a method for generation of channel state information in a wireless communication system are provided. The method includes transmitting, by an evolved Node B (eNB), a first reference signal to a plurality of User Equipments (UEs), receiving channel state information generated based on the first reference signal from the plurality of UEs, selecting candidate UEs to which wireless resources are to be allocated and transmitting second reference signals to the selected candidate UEs, receiving channel state information generated based on the second reference signals from the candidate UEs, and selecting final UEs, to which wireless resources are to be allocated, from the candidate UEs based on the channel state information generated based on the second reference signals, and transmitting control information for data reception to the selected final UEs.

PRIORITY

This application claims the benefit under 35 U.S.C. § 119(a) of a Koreanpatent application filed in the Korean Intellectual Property Office onJan. 16, 2012 and assigned Serial No. 10-2012-0004666, the entiredisclosure of which is hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a wireless mobile communication system.More particularly, the present invention relates to a method foreffectively transmitting and receiving a Channel StateInformation-Reference Signal (CSI-RS) and measuring interference.

2. Description of the Related Art

Current mobile communication systems are developing, beyond the initiallevel of providing voice-oriented services, into a high quality wirelesspacket data communication system in order to provide a data service anda multimedia service. To this end, various standardizationorganizations, such as the 3rd Generation Partnership Project (3GPP),the 3GPP2, and the Institute of Electrical and Electronics Engineers(IEEE), are preparing 3rd generation evolution mobile communicationsystem standards employing multiple access schemes using multi-carriers.Recently, various mobile communication standards, including Long TermEvolution (LTE) of the 3GPP, Ultra Mobile Broadband (UMB) of the 3GPP2,and 802.16m of the IEEE, have been developed in order to support a highspeed-high quality wireless packet data transmission service based on amultiple access scheme using a multi-carrier.

The current 3rd generation evolution mobile communication systems, suchas LTE, UMB, and 802.16m, are based on a multiple carrier multipleaccess scheme, employ multiple antennas based on a Multiple InputMultiple Output (MIMO) scheme, and use various technologies, such asbeam-forming, Adaptive Modulation and Coding (AMC), channel sensitivescheduling, and the like. These technologies improve the system capacityperformance by, for example, concentrating a transmission power ofmultiple antennas or controlling the quantity of data transmitted fromthe antennas according to the channel qualities and selectivelytransmitting data to a user having a good channel quality. Thesetechniques are based on the channel state information between a basestation or an evolved Node B (eNB) and a mobile station or a UserEquipment (UE). Therefore, an eNB or a UE needs to measure the channelstate between them, and a CSI-RS is used in the measurement. The eNBrefers to an apparatus for downlink transmission and uplink reception,which is located at a predefined position, and one eNB performstransmission and reception with respect to cells. In one mobilecommunication system, a plurality of eNBs are geographically scatteredand each eNB performs transmission and reception with respect to thecells.

A reference signal is a signal used for demodulation and decoding of areceived data symbol by measuring channel states, such as the intensityor distortion of a channel, the intensity of interference, Gaussiannoise, or the like, between an eNB and a UE. Furthermore, a receiver candetermine the channel state of a wireless channel between the receiverand a transmitter by measuring an intensity of a signal received throughthe wireless channel, which has been transmitted with a predefinedtransmission power by the transmitter. The measured channel state of thewireless channel is used by the receiver to determine a data rate whichthe receiver will request from the transmitter.

The resources of time, frequency, and transmission power are limited ina mobile communication system. Therefore, an increase in the quantity ofresources allocated to a reference signal may decrease the quantity ofresources that can be allocated to transmission of traffic channels andthus may reduce the absolute quantity of transmitted data. In this case,although the performances of channel measurement and estimation may beimproved, the reduced absolute quantity of transmitted data may ratherdecrease the performance of the entire system throughput. Therefore, inorder to obtain an optimum performance in view of the entire systemthroughput, a proper distribution between resources for the referencesignal and resources for transmission of traffic channels is necessary.

FIG. 1 illustrates transmission of various signals in a PhysicalResource Block (PRB) pair in a Long Term Evolution-Advanced (LTE-A)system according to the related art.

Referring to FIG. 1, one PRB pair includes 14 Orthogonal FrequencyDivision Multiplexing (OFDM) symbols along the time axis and 12subcarriers along the frequency axis. The 14 OFDM symbols and the 12subcarriers form 168 (=14×12) Resource Elements (REs), wherein each REcorresponds to a resource having an orthogonality with respect to aneighboring RE. In the PRB pair, a Physical Downlink Shared Channel(PDSCH) used for transmission of traffic data, a Cell-Specific ReferenceSignal (CRS) transmitted for each cell, a Physical Downlink ControlChannel (PDCCH) used for transmission of a control signal, aDemodulation Reference Signal (DMRS) used for reception of a PDSCH, anda CSI-RS used for measuring a downlink channel state and generatingchannel state information are allocated different REs for transmission.The CSI-RS supported in an LTE-A system can support signals for oneantenna port, 2 antenna ports, 4 antenna ports, and 8 antenna ports, andthe number of REs allocated in one PRB pair are different according tothe number of antenna ports of the CSI-RS as illustrated in FIG. 1.

FIG. 2 illustrates a transmission of a CSI-RS having four antenna portsin one PRB pair in an LTE-A system according to the related art.

Referring to FIG. 2, as indicated by reference numerals 200 and 210,sequences for four CSI-RS antenna ports are spread by orthogonal codes,Code-Division-Multiplexed (CDM), and transmitted to four REs. Thesequences for CSI-RS port 0 and CSI-RS port 1 are transmitted using thesequences for CSI-RS port 2 and CSI-RS port 3 and another RE pair. Inthis way, sequences for a plurality of CSI-RS antenna ports may betransmitted using a plurality of REs. In a case of an LTE-A system,since transmission to a maximum of 8 CSI-RS antenna ports is possible,an eNB can transmit CSI-RSs for a maximum of 8 transmission antennas.

In the case of an LTE-A system, transmission and reception can beperformed using CSI-RSs supporting a maximum of 8 CSI-RS transmissionantennas to one transmission point as described above. In a case ofperforming a beam forming transmission using a maximum of 8 transmissionantennas, a beam forming gain of a maximum of 9 dB is obtained, so as toimprove the Signal to Interference and Noise Ratio (SINR).

Therefore, a need exists for a method and an apparatus for transmittinga reference signal for effective data transmission and reception,measuring interference, and generating channel state information in aMIMO transmission and reception.

The above information is presented as background information only toassist with an understanding of the present disclosure. No determinationhas been made, and no assertion is made, as to whether any of the abovemight be applicable as prior art with regard to the present invention.

SUMMARY OF THE INVENTION

Aspects of the present invention are to address at least theabove-mentioned problems and/or disadvantages and to provide at leastthe advantages described below. Accordingly, an aspect of the presentinvention is to provide a method and an apparatus for transmitting areference signal for effective data transmission and reception,measuring interference, and generating channel state information in aMultiple Input Multiple Output (MIMO) transmission and reception.

Another aspect of the present invention is to provide an effectivetransmission and reception method in a MIMO system having scores or moretransmission antennas.

In accordance with an aspect of the present invention, a method oftransmitting a reference signal for generation of channel stateinformation by an evolved Node B (eNB) in a wireless communicationsystem is provided. The method includes transmitting a first referencesignal to a plurality of User Equipments (UEs), receiving channel stateinformation generated based on the first reference signal from theplurality of UEs, selecting candidate UEs to which wireless resourcesare to be allocated and transmitting second reference signals to theselected candidate UEs, receiving channel state information generatedbased on the second reference signals from the candidate UEs, andselecting final UEs, to which wireless resources are to be allocated,from the candidate UEs based on the channel state information generatedbased on the second reference signals, and transmitting controlinformation for data reception to the selected final UEs.

In accordance with another aspect of the present invention, a method ofreceiving a reference signal for generation of channel state informationby a UE in a wireless communication system is provided. The methodincludes receiving a first reference signal from an eNB, generatingchannel state information based on the first reference signal, andtransmitting the generated channel state information to the eNB,receiving a second reference signal from the eNB, generating channelstate information based on the second reference signal, and transmittingthe generated channel state information to the eNB, and receivingcontrol information for data reception from the eNB.

In accordance with another aspect of the present invention, an eNBapparatus for transmitting a reference signal for generation of channelstate information in a wireless communication system is provided. TheeNB apparatus includes a first reference signal transmitter forgenerating a first reference signal and for transmitting the firstreference signal to a plurality of UEs, a second reference signaltransmitter for receiving channel state information generated based onthe first reference signal from the plurality of UEs, for selectingcandidate UEs to which wireless resources are to be allocated, forgenerating second reference signals, and for transmitting the secondreference signals to the selected candidate UEs, a controller forcontrolling the first reference signal transmitter and the secondreference signal transmitter, for receiving channel state informationgenerated based on the second reference signals from the candidate UEs,for selecting final UEs, to which wireless resources are to beallocated, from the candidate UEs based on the channel state informationgenerated based on the second reference signals, and for transmittingcontrol information for data reception to the selected final UEs, amapper for mapping signals output from the first reference signaltransmitter, the second reference signal transmitter, and the controllerto wireless resources, and for transmitting the signals through thewireless resources.

In accordance with another aspect of the present invention, a UEapparatus for receiving a reference signal for generation of channelstate information in a wireless communication system is provided. The UEapparatus includes a first reference signal receiver for receiving afirst reference signal from an eNB and for generating channel stateinformation based on the first reference signal, a second referencesignal receiver for receiving a second reference signal from the eNB andfor generating channel state information based on the second referencesignal, a controller for receiving control information for datareception from the eNB and for controlling the first reference signalreceiver and the second reference signal receiver, and a demapper foridentifying the first reference signal, the second reference signal, anda control signal.

In accordance with another aspect of the present invention, a method oftransmitting a reference signal for generation of channel stateinformation by an eNB in a wireless communication system is provided.The method includes grouping a plurality of transmission antennas into aplurality of antenna groups, transmitting the reference signal to UEs indifferent time intervals or frequency intervals according to the antennagroups, and transmitting another reference signal for all of the antennagroups in an identical time interval or frequency interval.

In accordance with another aspect of the present invention, a method oftransmitting a reference signal for generation of channel stateinformation by an eNB in a wireless communication system is provided.The method includes grouping a plurality of beams transmitted through aplurality of transmission antennas into a plurality of beam groups,transmitting the reference signal to UEs in different time intervals orfrequency intervals according to the beam groups, and transmittinganother reference signal for all of the antenna groups in an identicaltime interval or frequency interval.

Other aspects, advantages, and salient features of the invention willbecome apparent to those skilled in the art from the following detaileddescription, which, taken in conjunction with the annexed drawings,discloses exemplary embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects, features, and advantages of certainexemplary embodiments of the present invention will be more apparentfrom the following description taken in conjunction with theaccompanying drawings, in which:

FIG. 1 illustrates a transmission of various signals in a PhysicalResource Block (PRB) pair in a Long Term Evolution-Advanced (LTE-A)system according to the related art;

FIG. 2 illustrates a transmission of a Channel StateInformation-Reference Signal (CSI-RS) having four antenna ports in onePRB pair in an LTE-A system according to the related art;

FIG. 3 illustrates a structure of a massive Multiple Input MultipleOutput (MIMO) system according to an exemplary embodiment of the presentinvention;

FIG. 4 illustrates an antenna grouping according to an exemplaryembodiment of the present invention;

FIG. 5 illustrates a transmission of CSI-RSs for massive MIMO accordingto an exemplary embodiment of the present invention;

FIG. 6 illustrates a transmission of CSI-RSs for massive MIMO byallocation of individual frequency resources according to an exemplaryembodiment of the present invention;

FIG. 7 illustrates a grouping of a plurality of beams into five beamgroups, wherein each beam group is transmitted in an individual timeinterval, according to an exemplary embodiment of the present invention;

FIG. 8 illustrates a transmission of CSI-RSs for a plurality of beams byallocating not only individual time resources but also individualfrequency resources according to an exemplary embodiment of the presentinvention;

FIG. 9 illustrates a link adaptation method according to an exemplaryembodiment of the present invention;

FIG. 10 illustrates a transmission of a first CSI-RS and a second CSI-RSin a frequency band according to an exemplary embodiment of the presentinvention;

FIG. 11 illustrates a transmission of first CSI-RSs and second CSI-RSsfor respective subframes according to an exemplary embodiment of thepresent invention;

FIG. 12 illustrates a notification of an allocation-or-not of a secondCSI-RS and an allocation of an interference measurement resource to aUser Equipment (UE) by an evolved Node B (eNB) according to an exemplaryembodiment of the present invention;

FIG. 13 illustrates an allocation of interference measurement resourcesin a frequency band according to an exemplary embodiment of the presentinvention;

FIG. 14 illustrates a transmission apparatus according to an exemplaryembodiment of the present invention; and

FIG. 15 illustrates a reception apparatus according to an exemplaryembodiment of the present invention.

Throughout the drawings, it should be noted that like reference numbersare used to depict the same or similar elements, features, andstructures.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

The following description with reference to the accompanying drawings isprovided to assist in a comprehensive understanding of exemplaryembodiments of the invention as defined by the claims and theirequivalents. It includes various specific details to assist in thatunderstanding but these are to be regarded as merely exemplary.Accordingly, those of ordinary skill in the art will recognize thatvarious changes and modifications of the embodiments described hereincan be made without departing from the scope and spirit of theinvention. In addition, descriptions of well-known functions andconstructions may be omitted for clarity and conciseness.

The terms and words used in the following description and claims are notlimited to the bibliographical meanings, but, are merely used by theinventor to enable a clear and consistent understanding of theinvention. Accordingly, it should be apparent to those skilled in theart that the following description of exemplary embodiments of thepresent invention is provided for illustration purpose only and not forthe purpose of limiting the invention as defined by the appended claimsand their equivalents.

It is to be understood that the singular forms “a,” “an,” and “the”include plural referents unless the context clearly dictates otherwise.Thus, for example, reference to “a component surface” includes referenceto one or more of such surfaces.

By the term “substantially” it is meant that the recited characteristic,parameter, or value need not be achieved exactly, but that deviations orvariations, including for example, tolerances, measurement error,measurement accuracy limitations and other factors known to those ofskill in the art, may occur in amounts that do not preclude the effectthe characteristic was intended to provide.

Furthermore, although the following detailed description of exemplaryembodiments of the present invention mainly discusses an OrthogonalFrequency Division Multiplexing (OFDM) based wireless communicationsystem, especially an Enhanced Universal Mobile TelecommunicationsSystem Terrestrial Radio Access (EUTRA) standard of the 3rd GenerationPartnership Project (3GPP), the principal idea of the present inventioncan be applied to other communication systems having similar technicalbackgrounds and channel types with slight modifications withoutdeparting from the scope of the present invention.

First, an exemplary method of effective transmission and reception of areference signal in a massive Multiple Input Multiple Output (MIMO)wireless communication system which transmits data by using scores ormore transmission antennas will be described.

FIGS. 3 through 15, discussed below, and the various exemplaryembodiments used to describe the principles of the present disclosure inthis patent document are by way of illustration only and should not beconstrued in any way that would limit the scope of the disclosure. Thoseskilled in the art will understand that the principles of the presentdisclosure may be implemented in any suitably arranged communicationssystem. The terms used to describe various embodiments are exemplary. Itshould be understood that these are provided to merely aid theunderstanding of the description, and that their use and definitions inno way limit the scope of the invention. Terms first, second, and thelike are used to differentiate between objects having the sameterminology and are in no way intended to represent a chronologicalorder, unless where explicitly stated otherwise. A set is defined as anon-empty set including at least one element.

FIG. 3 illustrates a structure of a massive MIMO system according to anexemplary embodiment of the present invention.

Referring to FIG. 3, a base station transmitter (i.e., an evolved Node B(eNB)) 300 transmits a wireless signal through scores or moretransmission antennas. The transmission antennas are arranged whilemaintaining a minimum distance (e.g., 0.5λ in FIG. 3) between each otheras indicated by reference numeral 310. For example, the minimum distancemay be one-half of the wavelength of the transmitted wireless signal. Ingeneral, when a distance corresponding to one-half of a wavelength of atransmitted wireless signal is maintained between transmission antennas,the signal transmitted from each transmission antenna is influenced by awireless channel having a low correlation. When a band of a transmittedwireless channel is 2 GHz, the minimum distance is 7.5 cm. As the bandof the transmitted wireless channel becomes higher than 2 GHz, thisdistance becomes shorter.

Referring to FIG. 3, the scores or more transmission antennas arrangedin the eNB 300 are used in transmitting a signal to one User Equipment(UE) or a plurality of UEs as indicated by reference numeral 320. Aproper precoding scheme is applied to a plurality of transmissionantennas, so as to enable simultaneous transmission to the plurality oftransmission antennas. At this time, one UE can receive one or morespatial channels. In general, the number of spatial channels which oneUE can receive is determined depending on the number of receptionantennas owned by the UE and the channel condition. However, in the caseof simultaneous transmission to a plurality of UEs as illustrated inFIG. 3, signals transmitted to different UEs may cause interference(i.e., a multi-user MIMO interference) between them, according to thecombination of precoding schemes. The multi-user MIMO interference hasan influence which increases in proportion to the number of UEssimultaneously receiving a signal from an eNB, and weakens the signalreceiving performance. More specifically, in a massive MIMO system asillustrated in FIG. 3, the multi-user MIMO interference is a main factorhaving an influence on the performance.

Therefore, in order to effectively implement a massive MIMO system, a UEshould precisely measure the channel state and the size of interferenceand transmit effective channel state information to an eNB by using themeasured information. Upon receiving the channel state information fromthe UE, the eNB determines UEs to which it will make a downlinktransmission, a data transmission speed for the transmission, and aprecoding scheme to be applied. Since the massive MIMO system includes alarge number of transmission antennas, an application of the referencesignal transmission method of the related art and its measuring methodused in the Long Term Evolution (LTE)/LTE-Advanced (LTE-A) system maycause performance degradation. Furthermore, the method of the relatedart can neither measure the multi-user MIMO interference precisely,which is caused by the simultaneous transmission to a plurality of UEs,nor reflect the interference in the channel state information.

Therefore, an exemplary embodiment of the present invention provides aneffective Channel State Information-Reference Signal (CSI-RS)transmission and reception method in a massive MIMO system.

In a massive MIMO transmission and reception based on an LTE/LTE-Asystem, an important subject is to maintain the backward compatibility.Here, the backward compatibility refers to a function capable oftransmitting or receiving a wireless signal to or from UEs of therelated art having no capability of receiving a massive MIMO signal,according to a scheme of the related art other than the MIMO scheme,simultaneously while operating a massive MIMO function in an LTE/LTE-Asystem. For example, the backward compatibility needs a capability ofsimultaneously transmitting a wireless signal to both a UE supportingmassive MIMO and a UE not supporting massive MIMO by using the samefrequency and time resources while preventing performance degradation ofthe UE supporting massive MIMO during the transmission or reception ofthe signal transmitted to the UE not supporting massive MIMO.

In order to satisfy the conditions as described above, an exemplaryembodiment of the present invention proposes a method of transmitting aCSI-RS for a massive MIMO system by using CSI-RS resources of therelated art introduced in the LTE-A. The CSI-RS resources of the relatedart introduced in the LTE-A support a maximum of 8 transmissionantennas. Therefore, in order to use the CSI-RS resources of the relatedart, a method capable of transmitting a signal through scores or moretransmission antennas is needed. In order to transmit a CSI-RS throughscores or more transmission antennas by using limited resources asdescribed above, an eNB divides the transmission antennas into aplurality of groups.

FIG. 4 illustrates an antenna grouping according to an exemplaryembodiment of the present invention.

Referring to FIG. 4, 40 transmission antennas (as indicated by referencenumeral 400), which have been grouped into five antenna groups, areillustrated as an example of a massive MIMO system. One antenna groupincludes 8 transmission antennas. Although the grouping is made based onactual transmission antennas in FIG. 4, the grouping may be made basedon virtual transmission antennas other than the actual transmissionantennas. Furthermore, the antenna grouping may be commonly applied toboth the actual transmission antennas and the virtual transmissionantennas. In general, a virtual transmission antenna refers to anindividual antenna signal which can be identified by a UE, and isimplemented by a signal transmitted from an actual transmission antenna.

CSI-RSs for massive MIMO are transmitted to each UE for each group ofmultiple antennas as illustrated in FIG. 4, so that CSI-RSs for moretransmission antennas than the 8 transmission antennas supported by theLTE-A are transmitted.

FIG. 5 illustrates transmission of CSI-RSs for massive MIMO according toan exemplary embodiment of the present invention.

Referring to FIG. 5, the CSI-RSs for massive MIMO are transmitted atdifferent time intervals according to the respective antenna groups inFIG. 4. In FIG. 5, one time interval corresponds to one subframe in anLTE/LTE-A system. The subframe is a time unit used for resourceallocation in an LTE/LTE-A system and corresponds to 1 msec. Forexample, antenna groups 1 through 5 in FIG. 4 are allocated transmissiontime intervals and transmit CSI-RSs in the allocated transmission timeintervals. In FIG. 4, since each antenna group includes 8 transmissionantennas, each antenna group transmits a CSI-RS in each transmissioninterval by using one CSI-RS resource (as indicated by reference numeral500 of FIG. 5) for 8 ports. In the transmission described above, eachtransmission antenna has an individual transmission resource intransmitting a CSI-RS, and the UE can measure the channel state in eachtransmission antenna. The channel state in each transmission antennaneeds to be measured for determination of an optimum precoding scheme inmassive MIMO. In contrast, UEs which do not support massive MIMO cannotreceive signals transmitted from a large number of antennas asillustrated in FIG. 4 in a discriminated manner according to theantennas. For such UEs, separately from the CSI-RS 510 for massive MIMO,CSI-RSs 520 for non-massive MIMO with respect to all antenna groups aretransmitted. The CSI-RSs 520 may be transmitted through a virtualantenna. Furthermore, the CSI-RSs for non-massive MIMO may also beeffectively used for simultaneous signal transmission to a plurality ofUEs. UEs which do not support massive MIMO can receive non-massive MIMOCSI-RSs for a maximum of 8 transmission antennas supported in the LTE-A.Such UEs cannot receive the CSI-RSs 510 for massive MIMO of FIG. 5 andmeasure the channel state of each transmission antenna. Therefore, forsuch UEs, the eNB implements a smaller number of virtual transmissionantennas than the number of actual transmission antennas and transmits asignal for these antennas through one CSI-RS resource. When CSI-RSs fornon-massive MIMO are transmitted for the UEs which do not supportmassive MIMO as described above, the UEs cannot discriminately measurethe channel state of the individual transmission antennas. However, aplurality of transmission antennas are allocated to each virtualtransmission antenna and a relatively higher transmission power is thusused for signal transmission. Furthermore, the higher the transmissionpower allocated to each virtual transmission antenna, the more precisethe channel state measured by a UE.

Although an individual time resource allocated to each antenna group isused for transmission of CSI-RSs for massive MIMO in FIG. 5, not onlythe individual time resource but also an individual frequency resourcemay be allocated for transmission of the CSI-RSs for massive MIMO.

FIG. 6 illustrates a transmission of CSI-RSs for massive MIMO byallocation of individual frequency resources according to an exemplaryembodiment of the present invention.

Referring to FIG. 6, CSI-RSs 600 for massive MIMO are transmitted in atime interval corresponding to one subframe. It is noted from FIG. 6that CSI-RSs 610 for a plurality of antenna groups are transmitted bydifferent OFDM symbols or subcarriers within the same subframe.

In order to transmit the CSI-RSs for massive MIMO, it is possible to usenot only the above two methods of allocating an individual time resourceor an individual frequency resource to each antenna group as illustratedin FIGS. 5 and 6 but also a method of allocating time and frequencyresources by complexly using the two methods.

In order to transmit the CSI-RSs for massive MIMO as illustrated inFIGS. 5 and 6, an eNB should notify a UE of related control informationbefore transmitting a CSI-RS. The control information is imperative forproper reception of the CSI-RSs for massive MIMO and properdetermination of the channel state based on the received CSI-RSs by theUE. Such control information may include at least one of the following:

1) Information on the number of transmission antennas which configurethe CSI-RSs for massive MIMO;

2) Information on the number of antenna groups which configure theCSI-RSs for massive MIMO;

3) Information on the number of transmission antennas which configureeach of the antenna groups configuring the CSI-RSs for massive MIMO;

4) Information on the time and frequency resource positions at which theCSI-RSs for massive MIMO are transmitted, wherein this informationincludes positions of time and frequency resources at which a CSI-RS foreach antenna group is transmitted;

5) Time period by which the CSI-RSs for massive MIMO are transmitted;

6) Information on the transmission power of the CSI-RSs for massiveMIMO, which includes a ratio between the transmission power of theCSI-RSs and the transmission power of a Physical Downlink Shared Channel(PDSCH), and the like; and

7) An initial state value used for generation of a scrambling sequenceof the CSI-RSs for massive MIMO.

Furthermore, in relation to the non-massive MIMO, the controlinformation may include at least one of the following:

1) Information on the number of transmission antennas which configurethe CSI-RSs for non-massive MIMO;

2) Information on the time and frequency resource positions at which theCSI-RSs for non-massive MIMO are transmitted;

3) Time period by which the CSI-RSs for non-massive MIMO aretransmitted;

4) Information on the transmission power of the CSI-RSs for non-massiveMIMO, which includes a ratio between the transmission power of theCSI-RSs and the transmission power of PDSCH, and the like; and

5) An initial state value used for generation of a scrambling sequenceof the CSI-RSs for non-massive MIMO.

The information on the transmission power of the CSI-RSs for massiveMIMO and the information on the transmission power of the CSI-RSs fornon-massive MIMO are control information used for a UE to receive eachCSI-RS and determine a precise channel state. Among the aboveinformation, the control information relating to the CSI-RSs for massiveMIMO and the control information relating to the CSI-RSs for non-massiveMIMO are transferred to the UE, separately from the CSI-RSs. Accordingto whether the information relates to massive MIMO or non-massive MIMO,different methods are used in determining the channel state. Therefore,for effective communication, the UE needs to know whether the above twotypes of information are for massive MIMO or non-massive MIMO. Forexample, the UE may receive both the control information relating to theCSI-RSs for massive MIMO and the control information relating to theCSI-RSs for non-massive MIMO, and the eNB may send additional controlinformation, by which it is possible to determine whether the controlinformation is for massive MIMO or for non-massive MIMO, to the UE.

The above description discusses a method of transmitting CSI-RSs formassive MIMO after dividing the CSI-RSs according to the antenna groups.In this method, a UE determines the channel state information bymeasuring the channel state of each antenna. Therefore, allocation ofindividual transmission resources is imperative for channel measurementof each antenna. This method can be effectively used when sufficienttransmission power can be allocated to each transmission antenna. Incontrast, when it is impossible to allocate a sufficient transmissionpower to each transmission antenna, it is more efficient to generate aplurality of beams by an eNB and select one or more beams among thegenerated beams by the UE, than to measure the channel state of eachantenna. In the method of using a plurality of beams as described above,each beam is transmitted using an individual transmission resource andbeams are generated using the same multiple transmission antennas, butdifferent precoding schemes are applied to the antennas according to thebeams. For example, although beam1 and beam2 are transmitted using thesame 40 transmission antennas, the precoding scheme applied to beam1 andthe precoding scheme applied to beam2 are different from each other.

When CSI-RSs for massive MIMO are transmitted using a plurality of beamstransmitted after being differently precoded, the plurality of beams maybe grouped into a plurality of beam groups for transmission, as in theabove case of grouping the multiple transmission antennas into multipleantenna groups for transmission.

FIG. 7 illustrates a grouping of a plurality of beams into five beamgroups, wherein each beam group is transmitted in an individual timeinterval, according to an exemplary embodiment of the present invention.

Referring to FIG. 7, five beam groups 710 are illustrated, each antennagroup includes 8 transmission antennas, and each antenna group transmitsCSI-RSs by using one CSI-RS resource 700 for 8 ports in eachtransmission interval. Although CSI-RSs for massive MIMO are transmittedusing a particular CSI-RS resource within a subframe as indicated byreference numeral 720, CSI-RSs for a large number of beams may betransmitted by performing transmission for different beam groupsaccording to subframes.

FIG. 8 illustrates a transmission of CSI-RSs for a plurality of beams byallocating not only individual time resources but also individualfrequency resources according to an exemplary embodiment of the presentinvention.

Referring to FIG. 8, CSI-RSs 800 for massive MIMO transmitted in a timeinterval corresponding to one subframe and individual frequencyresources 810 are illustrated.

Hereinafter, a link adaptation method in a massive MIMO system isdescribed.

For effective data transmission and reception using massive MIMO, it isimperative to efficiently use multi-user MIMO which simultaneouslytransmits a wireless signal to a plurality of UEs. A system usingmassive MIMO may have scores or more transmission antennas. In order touse such a large number of antennas, simultaneous transmission ofwireless signals to a large number of UEs is needed. In the case ofsimultaneously transmitting wireless signals to a large number of UEs, asignal for other UEs may generate a multi-user MIMO interference and thesize of the interference increases according to an increase in thenumber of UEs participating in the multi-user MIMO. For example, in thecase of performing multi-user MIMO for simultaneous transmission to 10UEs, one UE among the 10 UEs may be subjected to multi-user MIMOinterference by the signals transmitted from the other 9 UEs, whichcauses performance degradation of the one UE.

Furthermore, since signal transmission to a large number of UEs issimultaneously performed, it may be necessary to use, in spite of anoptimum precoding in view of a particular UE, another precoding inconsideration of the quantity of interference incurred to another UE. Ina case of an LTE/LTE-A system, a UE notifies an eNB of a precodingoptimum for the UE together with information on supportable data rateswhen the optimum precoding is applied. Since the information on thesupportable data rates is available only when the precoding is applied,it is impossible to know the data rates which the UE can support whenthe eNB applies a precoding which is not requested by the UE. Ingeneral, this problem is known as an inaccuracy of the link adaption.

An exemplary embodiment of the present invention proposes a linkadaptation method for addressing the problems occurring due to theinaccuracy of the link adaption as described above.

FIG. 9 illustrates a link adaptation method according to an exemplaryembodiment of the present invention.

Referring to FIG. 9, an eNB transmits a CSI-RS (i.e., a coarse CSI) 900for first channel measurement to a UE. Upon receiving the CSI-RS, the UEnotifies the eNB of first channel state information 910 by using theCSI-RS. When the CSI-RS 900 for the first channel measurement is aperiodic signal, the first channel state information 910 may also beperiodically notified information. The first channel state information910 may be notified of by each of multiple UEs. By using the notifiedfirst channel state information as described above, the eNB firstdetermines UEs to which wireless resources for data transmission are tobe allocated, in the step indicated by reference numeral 920. Theselected UEs are known as wireless resource allocation candidate UEs. Inthe step indicated by reference numeral 930, the wireless resourceallocation candidate UEs determined by the eNB in the step indicated byreference numeral 920 are notified that they should receive secondCSI-RSs from the eNB. The wireless resource allocation 920 and thesecond CSI-RS notification 930 may be simultaneously performed in thesame time interval. In the step indicated by reference numeral 940, theUE having received the second CSI-RS (i.e., fine CSI-RS) notifies theeNB of second channel state information by using the second CSI-RS. Uponreceiving the second channel state information, the eNB selects UEs towhich actual downlink wireless resources are to be allocated, andtransmits control information needed for reception of a traffic channelto the selected UEs, in the step indicated by reference numeral 950. TheUEs allocated the actual downlink wireless resources may be differentfrom the wireless resource allocation candidate UEs.

The second CSI-RS is different from the first CSI-RS in view of at leastone of the following:

1) The first CSI-RS is a signal simultaneously received by a pluralityof UEs, while different signals according to UEs are allocated andtransmitted as the second CSI-RSs;

2) The first CSI-RS is a signal which is periodically transmitted andreceived by a plurality of UEs, while the second CSI-RS is received byonly some of the UEs having received the first CSI-RS and the eNBdetermines whether to transmit the second CSI-RS;

3) The first CSI-RS is transmitted through all frequency bands, in orderto enable a UE to measure all frequency bands and find the bestfrequency band. In contrast, the second CSI-RS is transmitted in fewerthan all of the frequency bands according to a determination of the eNB,because the eNB has already found a frequency band most proper for theUE; and

4) A UE having measured the first CSI-RS determines an optimum precodingbased on the first CSI-RS. In contrast, the second CSI-RS does notrequire a process of determining an optimum precoding since it is asignal to which a precoding determined to be optimum for a correspondingUE by the eNB has been already applied.

The second channel state information transmitted by the UE havingreceived the second CSI-RS may be reported by means of a value relativeto the first channel state information. For example, if a Signal toInterference and Noise Ratio (SINR) or a data rate among the firstchannel state information is A and an SINR or a data rate measured by aUE having received a second CSI-RS is (A+Δ), the UE does not notify(A+Δ) but notifies only Δ as the second channel state information. Suchtransmission of a relative value as the channel state information asdescribed above reduces the information quantity of the second channelstate information, so as to reduce the overhead in the uplinktransmission by the UE.

FIG. 10 illustrates a transmission of a first CSI-RS and a second CSI-RSin a frequency band according to an exemplary embodiment of the presentinvention.

Referring to FIG. 10, the first CSI-RS 1000 is a signal which istransmitted in all Resource Blocks (RBs) of the system bandwidth and isreceived by a plurality of UEs. In contrast, the second CSI-RSs 1010,1020, 1030, and 1040 are signals, which may be individually allocated toUEs and may be transmitted in only some of the RBs of the systembandwidth. Furthermore, as illustrated in FIG. 10, a plurality of secondCSI-RSs may be transmitted in the same subframe and RB. For example,although the second CSI-RSs 1010 and 1020 are signals for different UEs,they are transmitted using the same RB in the same subframe.

In order to receive the second CSI-RSs, the eNB should transfer controlinformation for receiving the second CSI-RSs to the UE. The controlinformation for receiving the second CSI-RSs may be notified to the UEby the eNB through transmission as indicated by reference numeral 920 inFIG. 9. The control information includes at least one of the following:

1) Information on a UE to which the second CSI-RS corresponds. Thisinformation may be transmitted either by defining separate controlinformation or by initializing a Cyclic Redundancy Check (CRC) code of acontrol channel into UE-specific indicator information (UE ID);

2) Information on a frequency band (i.e., an RB) to which the secondCSI-RS is transmitted;

3) Information on a time interval (i.e., a subframe) to which the secondCSI-RS is transmitted;

4) Information on a CSI-RS transmission resource used for transmissionof the second CSI-RS when a plurality of CSI-RS transmission resourcesexists within the RB and the subframe through which the second CSI-RS istransmitted;

5) Information on the number of transmission antenna ports through whichthe second CSI-RS is transmitted; and

6) Ratio of transmission power between the second CSI-RS and a PDSCHtransmitted for a data signal.

The control information is imperative for reception of a second CSI-RSallocated to a UE by the UE. Moreover, the eNB may notify acorresponding UE of information needed for reception of a second CSI-RSallocated to UEs other than the corresponding UE. The reason why the eNBnotifies a UE of information needed for reception of a second CSI-RSallocated to the other UEs is in order to enable the UE to measure themulti-user MIMO interference generated at the time of multi-user MIMOtransmission by receiving the second CSI-RS allocated to the other UEs.In order to receive the second CSI-RS allocated to the other UEs formeasurement of the multi-user MIMO interference, information imperativefor reception of the second CSI-RS allocated to the other UEs is neededas is the information imperative for reception of the second CSI-RSallocated to the UE itself. The information imperative for reception ofthe second CSI-RS may be transmitted through a Physical Downlink ControlChannel (PDCCH) or an Enhanced-PDCCH (E-PDCCH), which are controlchannels supported in the LTE/LTE-A. Notification of all informationrelating to the second CSI-RS to a UE by using a PDCCH or an E-PDCCH asdescribed above may generate an excessive downlink overhead. In order toavoid such an excessive downlink overhead, some of the information maybe set using a higher layer signaling while only indispensibleinformation is transmitted using a PDCCH or an E-PDCCH.

Furthermore, the second CSI-RS of FIG. 10 is not transmitted in allfrequency bands but is transmitted in fewer than all of the frequencybands. The reason why the second CSI-RS is transmitted in fewer than allof the frequency bands is to transmit the second CSI-RS in the same bandas the frequency band in which a data signal is transmitted.Accordingly, it is possible to precisely determine the channel state ofthe frequency band in which the data is transmitted. One method capableof reducing the quantity of control information for the second CSI-RS,which should be transmitted through a PDCCH or an E-PDCCH, is tosemi-statically set the transmission resources for the second CSI-RS.

FIG. 11 illustrates a transmission of first CSI-RSs and second CSI-RSsfor respective subframes according to an exemplary embodiment of thepresent invention.

Referring to FIG. 11, first CSI-RSs and second CSI-RSs aresimultaneously transmitted in subframe 0. In subframe 0, both the firstCSI-RSs and second CSI-RSs are transmitted using CSI-RS transmissionresources 1100, 1110, 1120, and 1130. In subframe 7, only the secondCSI-RSs are transmitted using CSI-RS transmission resources 1140, 1150,1160, and 1170. Furthermore, it is noted that the CSI-RS transmissionresources 1100, 1110, 1120, and 1130 have been allocated to UEsappointed by the eNB. For example, the CSI-RS transmission resource 1100has been allocated to enable UEs belonging to Group A to receive thesecond CSI-RS. For example, when a UE belonging to Group A receives anotification from the eNB that a second CSI-RS for the UE itself isallocated to a particular RB or RBs, the UE can identify a CSI-RStransmission resource in which the second CSI-RS for the UE itselfexists, among a plurality of CSI-RS transmission resources existing inthe particular RB or RBs. This method can reduce the downlink overheadbecause it makes it unnecessary for the UE to transmit, through a PDCCHor an E-PDCCH, separate control information about which one is allocatedamong a plurality of CSI-RS transmission resources. Furthermore, when aUE belonging to Group A in FIG. 11 receives a notification from the eNBthat a second CSI-RS for the UE itself is allocated to a particularRB(s), the UE can identify that the second CSI-RS for the UE itselfexists in the CSI-RS transmission resource 1100 and that CSI-RSs forother UEs exist in the other CSI-RS transmission resources 1110, 1120,and 1130. By using this information, the UE can determine a multi-userMIMO interference generated in the same RB(s) as that of the UE itselfby measuring the reception power carried by the CSI-RS transmissionresources 1110, 1120, and 1130.

When predefined CSI-RS resources are set for the second CSI-RSs asillustrated in FIG. 11, at least one of the following should be notifiedto a UE by using a higher layer signaling:

1) Information on CSI-RS transmission resources for the second CSI-RSsto be allocated to the UE; and

2) Information on CSI-RS transmission resources for the second CSI-RSsto be allocated to other UEs.

When the UE having received the information described above receives anotification that a second CSI-RS has been allocated to itself through aPDCCH or an E-PDCCH, the UE receives a signal for measurement of awireless channel in the CSI-RS transmission resource through which thesecond CSI-RS of the UE itself is transmitted and receives a signal formeasurement of a multi-user MIMO interference in the other CSI-RStransmission resource.

Hereinafter, an exemplary method of measuring an interference signal ina massive MIMO system will be described.

For effective data transmission and reception using massive MIMO, a UEneeds to effectively determine a multi-user MIMO interference generatedin massive MIMO transmission and reception. Although the abovedescription proposes an exemplary method of measuring a multi-user MIMOinterference by using second CSI-RSs allocated to other UEs, exemplaryembodiments of the present invention can be applied to a method ofmeasuring a multi-user MIMO interference by directly measuring secondCSI-RSs allocated to other UEs and a method of measuring a multi-userMIMO interference by allocating an interference measurement resource toeach UE to which a second CSI-RS is allocated.

The interference measurement resource refers to a wireless resource usedwhen a particular UE measures the size of interference applied to theparticular UE itself, and is used when a UE has received a second CSI-RSand needs to determine a precise channel state information. Theinterference measurement resource includes one or more REs, throughwhich a wireless signal transmitted to a UE allocated the interferencemeasurement resource is not carried and only wireless signalstransmitted to the other UEs are carried. For example, when UE1 has beenallocated a particular interference measurement resource, the eNBtransmits only transmission signals for the other UEs withouttransmitting a transmission signal for UE1 through the particularinterference measurement resource, so that UE1 can measure only themulti-user MIMO interference. The UE having received only thetransmission signals for the other UEs through the particularinterference measurement resource can measure a precise multi-user MIMOinterference.

Each UE may be notified of whether the interference measurement resourcehas been allocated or not, through a PDCCH or an E-PDCCH. In this case,the eNB may notify allocation of the interference measurement resourceto the UE simultaneously while notifying whether the second CSI-RS hasbeen allocated.

FIG. 12 illustrates a notification of an allocation-or-not of a secondCSI-RS and an allocation of an interference measurement resource to a UEby an eNB according to an exemplary embodiment of the present invention.

Referring to FIG. 12, it is the same as FIG. 9 except for the partindicated by reference numeral 1230. For example, in FIG. 12, an eNBtransmits a CSI-RS (i.e., a coarse CSI) 1200 for first channelmeasurement to a UE. Upon receiving the CSI-RS, the UE notifies the eNBof first channel state information 1210 by using the CSI-RS. When theCSI-RS 1200 for the first channel measurement is a periodic signal, thefirst channel state information 1210 may also be periodically notifiedinformation. The first channel state information 1210 may be notified ofby each of multiple UEs. By using the notified first channel stateinformation as described above, the eNB selects wireless resourceallocation candidate UEs, in the step indicated by reference numeral1220. In the step indicated by reference numeral 1230, the wirelessresource allocation candidate UEs determined by the eNB in the stepindicated by reference numeral 1220 are notified that they shouldreceive second CSI-RSs from the eNB. Furthermore, in the step indicatedby reference numeral 1220, the eNB notifies allocation of the secondCSI-RS and the interference measurement resource to the UE. The wirelessresource allocation 1220 and the second CSI-RS notification 1230 may besimultaneously performed in the same time interval. In the stepindicated by reference numeral 1240, the UE having received the secondCSI-RS (i.e., fine CSI-RS) notifies the eNB of second channel stateinformation by using the second CSI-RS. Upon receiving the secondchannel state information, the eNB selects UEs to which actual downlinkwireless resources are to be allocated, and transmits controlinformation needed for reception of a traffic channel to the selectedUEs, in the step indicated by reference numeral 1250.

The control information for notification of the interference measurementresource transmitted in the step indicated by reference numeral 1230includes at least one of the following:

1) Information on a UE to which the interference measurement resourcecorresponds. This information may be transmitted either by definingseparate control information or by initializing a CRC code of a controlchannel into UE-specific indicator information (UE ID);

2) Information on a frequency band (i.e., an RB) in which theinterference measurement resource exists;

3) Information on a time interval (i.e., a subframe) in which theinterference measurement resource exists; and

4) Information on a CSI-RS transmission resource to be used fortransmission of the interference measurement resource when a pluralityof interference measurement resources exists within the RB and thesubframe through which the interference measurement resource istransmitted.

In addition to the exemplary method illustrated in FIG. 12 in which aninterference measurement resource is allocated using a control channel,such as a PDCCH or an E-PDCCH, there is an exemplary method in which aninterference measurement resource of a fixed position is allocated usinga higher layer signaling. In this exemplary method, when a UE isallocated a second CSI-RS, the UE uses an interference measurementresource set through a higher layer signaling among RBs in which thesecond CSI-RSs exist. This exemplary method is advantageous in that itis not necessary to transmit separate control information through aPDCCH or an E-PDCCH in order to allocate an interference measurementresource. Another exemplary method is to link an interferencemeasurement resource and a CSI-RS transmission resource of a secondCSI-RS. In this exemplary method, an interference measurement resourceallocated to a UE becomes different according to the CSI-RS transmissionresource used by the second CSI-RS allocated to the UE.

FIG. 13 illustrates an allocation of interference measurement resourcesin a frequency band according to an exemplary embodiment of the presentinvention.

Referring to FIG. 13, the first CSI-RS 1300 is a signal which istransmitted in all RBs of the system bandwidth and is received by aplurality of UEs. A specific second CSI-RS and a specific interferencemeasurement resource have been allocated to each of the two UEs in thetwo same RBs. UE1 measures the channel state of the wireless channel byusing the second CSI-RSs 1310 and measures the quantity of interferenceby using the interference measurement resources 1320. UE2 measures thechannel state of the wireless channel by using the second CSI-RSs 1330and measures the quantity of interference by using the interferencemeasurement resources 1340. It is noted that the interferencemeasurement resources and the second CSI-RSs occupy the same frequencyband. Such occupying of the same frequency band is intended to enablechannel estimation and interference measurement to be performed in thefrequency band in which actual data is to be transmitted, so as toobtain a more precise determination of the channel state information.Furthermore, the interference measurement resources and the secondCSI-RSs are not transmitted in all frequency bands but are transmittedin fewer than all of the frequency bands. Such transmission in fewerthan all of the frequency bands is also performed in only the same bandas a particular frequency band in which a data signal is to betransmitted, so as to obtain a more precise determination of the channelstate information of the particular frequency band in which the datasignal is to be transmitted.

FIG. 14 illustrates a transmission apparatus according to an exemplaryembodiment of the present invention.

Referring to FIG. 14, a controller 1410 determines whether to transmit asignal generated in a first CSI-RS transmitter 1400 and a signalgenerated in a second CSI-RS/interference measurement resourcetransmitter 1420. As described above, the first CSI-RS signal is aperiodically transmitted signal and is transmitted in order to measurechannels for a plurality of transmission antennas or a plurality ofbeams generated by a plurality of transmission antennas. In contrast, inthe case of the second CSI-RS, the eNB determines, in each subframe, atime interval in which the second CSI-RS is to be transmitted, a UE towhich the second CSI-RS is to be transmitted, and a frequency band inwhich the second CSI-RS is to be transmitted. Thereafter, the signal istransmitted while being carried by a Resource Element (RE) to betransmitted by an RE mapper 1430. Furthermore, the controller 1410notifies control information on transmission of the second CSI-RS andthe interference measurement resource to each UE by using a PDCCH or anE-PDCCH. Here, the information needed for reception of the second CSI-RSor the interference measurement resource by each UE may include a partwhich the UE can determine according to a predefined rule between the UEand an eNB.

FIG. 15 illustrates a reception apparatus according to an exemplaryembodiment of the present invention.

Referring to FIG. 15, received wireless signals are classified intofirst CSI-RSs, second CSI-RSs, and interference measurement resources byan RE demapper 1500, which are then input to a first CSI-RS receiver1510 and a second CSI-RS/interference measurement resource receiver1530, respectively. The first CSI-RS receiver 1510 is a receiver forreceiving signals transmitted in all frequency bands, and the secondCSI-RS/interference measurement resource receiver 1530 is a receiver forreceiving only signals transmitted in the time intervals and frequencybands allocated by the eNB. A controller 1520 determines the timeinterval and the frequency band in which the second CSI-RS/interferencemeasurement resource receiver 1530 is to receive a signal. Thecontroller 1520 is notified of the determination by receiving a PDCCH oran E-PDCCH from the eNB or identifies the corresponding informationbased on a predefined rule between the eNB and the UE as describedabove.

According to exemplary embodiments of the present invention, it ispossible to effectively transmit a reference signal in a MIMO systemhaving scores or more transmission antennas. Furthermore, according toexemplary embodiments of the present invention, it is possible toallocate resources to a reference signal and measure an interferencesignal in a MIMO system having scores or more transmission antennas.

While the invention has been shown and described with reference tocertain exemplary embodiments thereof, it will be understood by thoseskilled in the art that various changes in form and details may be madetherein without departing from the spirit and scope of the presentinvention as defined by the appended claims and their equivalents.

What is claimed is:
 1. A method for receiving reference signals by auser equipment (UE) in a wireless communication system, the methodcomprising: receiving control information indicating a plurality ofresources that are configured to the UE for the reference signals;receiving, based on the control information, the reference signalstransmitted on the plurality of resources using a beam group from a basestation; and transmitting channel state information generated based onthe received reference signals to the base station, wherein theplurality of resources comprises a plurality of resource elements (REs)allocated based on a number of antenna ports for transmission of thereference signals, and wherein a plurality of beams for the antennaports are grouped into a plurality of beam groups, and the plurality ofbeam groups have different beam directions and are mapped to differentREs.
 2. The method of claim 1, wherein the control information comprisesat least one of: information indicating a number of the antenna portsconfigured for each of the beam groups; or information indicating theplurality of resources on a time and frequency domain on which thereference signals for each beam group are transmitted.
 3. The method ofclaim 1, wherein the control information further comprises at least oneof: information indicating a number of antenna ports configured for thereference signals; or information indicating a number of the beam groupsconfigured for the reference signals.
 4. The method of claim 1, furthercomprising: receiving downlink control information comprising firstinformation indicating at least one first resource that is configured tothe UE for at least one additional reference signal and secondinformation indicating at least one second resource that is configuredto the UE for interference measurement, on a physical downlink controlchannel; and receiving the at least one additional reference signal onthe at least one first resource, wherein the first information indicatesat least one of a frequency band on which the at least one additionalreference signal is transmitted, or a time interval in which the atleast one additional reference signal is transmitted aperiodically, andwherein the second information indicates at least one of a frequencyband on which the at least one second resource exists, or a timeinterval in which the at least one second resource exists.
 5. A methodfor transmitting reference signals by a base station in a wirelesscommunication system, the method comprising: transmitting controlinformation indicating a plurality of resources that are configured to auser equipment (UE) for the reference signals; transmitting thereference signals to the UE on the plurality of resources using a beamgroup among a plurality of beam groups; and receiving channel stateinformation from the UE from the UE after transmitting the referencesignals, wherein the plurality of resources comprises a plurality ofresource elements (REs) allocated based on a number of antenna ports fortransmission of the reference signals, and wherein a plurality of beamsfor the antenna ports are grouped into the plurality of beam groups, andthe plurality of beam groups have different beam directions and aremapped to different REs.
 6. The method of claim 5, wherein the controlinformation comprises at least one of: information indicating a numberof the antenna ports configured for each of the beam groups; orinformation indicating the plurality of resources on a time andfrequency domain on which the reference signals for each beam group istransmitted.
 7. The method of claim 6, wherein the control informationfurther comprises at least one of: information indicating a number ofantenna ports configured for the reference signals; or informationindicating a number of the beam groups configured for the referencesignals.
 8. The method of claim 5, further comprising: transmittingdownlink control information comprising first information indicating atleast one first resource that is configured to the UE for at least oneadditional reference signal and second information indicating at leastone second resource that is configured to the UE for interferencemeasurement, on a physical downlink control channel; and transmittingthe at least one additional reference signal on the at least one firstresource, wherein the first information indicates at least one of afrequency band on which the at least one additional reference signal istransmitted, or a time interval in which the at least one additionalreference signal is transmitted aperiodically, and wherein the secondinformation indicates at least one of a frequency band on which the atleast one second resource exists, or a time interval in which the atleast one second resource exists.
 9. A user equipment (UE) for receivingreference signals in a wireless communication system, the UE comprising:a transceiver configured to: receive control information indicating aplurality of resources that are configured to the UE for the referencesignals, receive the reference signals transmitted on the plurality ofresources using a plurality of beam groups from a base station, based onthe control information, and transmit channel state informationgenerated based on the received signals to the base station; and atleast one processor configured to control the transceiver, wherein theplurality of resources comprises a plurality of resource elements (REs)allocated based on a number of antenna ports for transmission of thereference signals, and wherein a plurality of beams for the antennaports are grouped into the plurality of beam groups, and the pluralityof beam groups have different beam directions and are mapped todifferent REs.
 10. The UE of claim 9, wherein the control informationcomprises at least one of: information indicating a number of thetransmission antennas configured for each of the beam groups; orinformation indicating the plurality of resources on a time andfrequency domain on which the reference signals for each beam group aretransmitted.
 11. The UE of claim 10, wherein the control informationfurther comprises at least one of: information indicating a number ofantenna ports configured for the reference signals; or informationindicating a number of the beam groups configured for the referencesignals.
 12. The UE of claim 9, wherein the transceiver is furtherconfigured to: receive downlink control information comprising firstinformation indicating at least one first resource that is configured tothe UE for at least one additional reference signal and secondinformation indicating at least one second resource that is configuredto the UE for interference measurement, on a physical downlink controlchannel; and receive the at least one additional reference signal on theat least one first resource, wherein the first information indicates atleast one of a frequency band on which the at least one additionalreference signal is transmitted, or a time interval in which the atleast one additional reference signal is transmitted aperiodically, andwherein the second information indicates at least one of a frequencyband on which the at least one second resource exists, or a timeinterval in which the at least one second resource exists.
 13. A basestation for transmitting reference signals in a wireless communicationsystem, the base station comprising: at least one processor configuredto generate control information indicating a plurality of resources thatare configured to a user equipment (UE) for the reference signals; and atransceiver configured to: transmit the control information to the UE,transmit the reference signals to the UE on the plurality of resourcesusing a beam group among a plurality of beam groups, and receive channelstate information from the UE after transmitting the reference signals,wherein the plurality of resources comprises a plurality of resourceelements (REs) allocated based on a number of antenna ports fortransmission of the reference signals, and wherein a plurality of beamsfor the antenna ports are grouped into the plurality of beam groups, andthe plurality of beam groups have different beam directions and aremapped to different REs.
 14. The base station of claim 13, wherein thecontrol information comprises at least one of: information indicating anumber of the antenna ports configured for each of the beam groups; orinformation indicating the plurality of resources on a time andfrequency domain on which the reference signals for each beam group istransmitted.
 15. The base station of claim 14, wherein the controlinformation further comprises at least one of: information indicating anumber of antenna ports configured for the reference signals; orinformation indicating a number of the beam groups configured for thereference signals.
 16. The base station of claim 13, wherein thetransceiver is further configured to: transmit downlink controlinformation comprising first information indicating at least one firstresource that is configured to the UE for at least one additionalreference signal and second information indicating at least one secondresource that is configured to the UE for interference measurement, on aphysical downlink control channel, and transmit the at least oneadditional reference signal on the at least one first resource, whereinthe first information indicates at least one of a frequency band onwhich the at least one additional reference signal is transmitted, or atime interval in which the at least one additional reference signal istransmitted aperiodically, and wherein the second information indicatesat least one of a frequency band on which the at least one secondresource exists, or a time interval in which the at least one secondresource exists.